A femto cell will be described in brief as follows.
Femto is a prefix in the metric system denoting a factor of 10-5. In this context, a femto cell refers to an indoor base station which is very small, consumes small power, and is used at homes or in the offices. The femto cell is used to have a meaning similar to a picocell, but it is generally admitted to have an evolved function.
The femto cell may be also called a femto base station (FBS) or a femto base transceiver station (BTS).
The femto BS, which is a small version of a macro base station and performs a majority of the functions of the macro base station, may be installed in an area covered by the macro base station or may be installed in a shadow area not covered by the macro base station. The femto base station has an independently operating network constitution and a far larger number of femto base stations than relay base stations may be installed in downtown areas or indoor areas.
The femto cell has the following advantages.
1. A cell coverage improvement
2. Infrastructure cost decrease
3. New service offering
4. Fixed mobile convergence (FMC) acceleration
One or more femto cells may be grouped by particular services or areas to constitute a femto cell group. For example, a femto cell group allowing only a particular terminal to access may be called a closed subscriber group (CSG). The femto BS checks a CSG identifier (CSG ID) of a terminal and allows only a terminal, which has subscribed to the CSG, to access.
FIG. 1 is a view illustrating the configuration of a wireless communication system including a femto BS.
As shown in FIG. 1, the wireless communication system including a femto BS may include a femto BS 110, a macro BS, a femto network gateway (FNG) 130, an access service network (ASN) 140, and at least one connectivity service network (CSN) 150. The macro BS refers to a general BS of the related art wireless communication system. The ASN 140 and CSN 150 may be connected by a link R3. Multiple CSNs may be connected by a link R5.
The femto BS 110 is on the premise that it directly accesses a TCP/IP (Transmission Control Protocol/Internet Protocol) network to independently operate like a macro BS and has a coverage ranging from 0.1 m to 30 m. One femto BS 110 is assumed to accommodate ten to twenty UEs. The femto BS 110 may use the same frequency as that of the macro BS (in case of Intra FA) or may use a different frequency (In case of Inter FA).
The femto BS 110 may be connected to the macro BS through an R1 interface to receive a downlink channel from the macro BS, or may transmit a control signal to the macro BS.
The femto BS 110 may cover an indoor area or a shadow area not covered by the macro BS and may be able to support a high data transmission rate. The femto BS 110 may be installed in an overlay manner within a macro cell or may be installed in a non-overlay manner in an area not covered by the macro BS. The femto BS 110 is classified into two types of femto BSs.
The first type of femto BS is a closed subscriber group (CSG) femto BS, and the second type of femto BS is an open subscriber group (OSG) femto BS. The CSG femto BS groups UES accessible thereto and gives a CSG ID (Identification) to the UEs, thus discriminating the UEs which have been assigned the CSG ID and UEs which are otherwise when the UEs access the CSG femto cell BS. The OSG femto cell BS is a BS allowing every UE to access.
The FNG 130 is a gateway that controls the femto BS 110, which is connected to the ASN 140 and the CSN 150 through an Rx interlace and an Ry interlace. The femto BS 110 may receive a service from the CSN 150 through the FNG 130, and a UE connected to the femto BS 110 may receive a service of a function such as authentication, IMS, and the like, from the FNG 130 of the CSN 150.
The CSN 150 may provide a connection of an application service such as the Internet, VoIP, and the like, to the UE and provide an authentication and billing (or charging) function, and the ASN 140 may control the macro BS and manage a connection between the macro BS and the CSN 150.
Meanwhile, the foregoing CSG type femto BS may be classified into two types of femto BSs according to the possibility of a connection of a non-member UE, namely, a UE which has not assigned the CSG ID.
Namely, a CSG closed femto ABS is a type of BS allowing only member UE to access. The UE may store CSG closed femto BS IDs allowing for its connection in a white list.
Meanwhile, a CSG open femto ABS preferentially supports a service of member UEs, and when there is leeway in the resources, the CSG open femto ABS allows non-member UEs to access. In this case, however, a service level with respect to the non-member UEs may be discriminated over the member UEs.
Currently, the mobile communication system using a femto BS is on the premise that the two types of femto BSs as to whether non-member UEs are allowed for accessing are fixed as described above with respect to the CSG type femto BS, and the differentiation of the CSG open/closed femto BSs may be recognized through partition information in an AAI_SCD (Advanced Air Interface System Configuration Descriptor) message.
Meanwhile, when the macro BS and the femto BS are installed in the same frequency domain, non-member CSG UEs connected to the macro BS may be affected by interference from neighboring femto BSs. In this case, the macro BS may not allow the corresponding femto BS not to use a certain resource domain, thus adjusting the interference to the UEs.
FIG. 2 is a flow chart illustrating the process of instructing, by the macro BS, femto cells to perform snooping according to the related art.
First, in a state in which the UE is connected to the macro BS (step S201), namely, after the UE performs a network entry toward the macro BS, it performs initial scanning (step S202).
When a femto preamble is detected according to the scanning performing results of the UE, the UE transmits an AAI_NBR-REQ message to the macro BS in order to request information regarding a neighbor BS (step S203).
In this case, when the UE stores the CSG ID in CSG white list, it may deliver the CSG ID together to request an accessible neighboring femto list.
The UE may receive a neighbor advertisement message (AAI_NBR-ADV message) from the macro BS in response to the NBR-REQ message (step S204).
When the NBR-REQ message is received from the UE, the macro BS may instruct femto BSs corresponding to the CSG ID to perform snooping through a backbone network (step S205).
Here, snooping refers to a method of allowing a femto BS to eavesdrop an uplink signal of the UE allowed to access the femto BS to thus recognize the UE by the femto BS.
The UE ID is included in the message instructing the snooping, so the femto BS can recognize the UE to be eavesdropped.
When a signaling value received from an adjacent UE is higher than a threshold value, the femto BS respond accordingly to the macro BS through a report.
Next, the macro BS may deliver a list of femto ABSs to be scanned (including a list of CSG femto BSs allowed for the UE's access) to acquire additional information to the UE through a nonsolicited scan response (unsolicited AAI_SCN-RSP) message (step S206).
In the related art as described above with reference to FIG. 2, when the UE detects a femto preamble, it can recognize the presence of a femto BS nearby, but it cannot ascertain whether or not the femto BS belongs to a CSG of the UE so the femto BS is allowed for the UE to access until when it obtains additional information such as BSID or a CSGID of the corresponding femto BS.
Thus, if the process of acquiring, by the BS, the list of femto BSs corresponding to the CSG ID through a network is additionally performed, it would take much time for the UE to discover a femto BS because the points in time when the AAI_NBR-REQ message or the AAI_SCN-REQ message is transmitted and the AAI_SCN-RSP message is received are delayed.
Namely, the terminal is anticipated to perform initial scanning when scanning conditions are met, and after the CSG ID is delivered through the NBR-REQ message, the macro BS will instruct snooping to instruct macro BS initiated handover (HO).
Namely, snooping of the femto BS may not start until the UE delivers the AAI_NBR-REQ or the AAI_SCN-REQ including the CSG ID (namely, after the UE already discovers the femto BS), so delay may occur until when the UE actually detects the femto BS.